1. Field of the Invention
The present invention relates generally to sound attenuation devices, and particularly to an expandable chamber acoustic silencer having a variable cross-sectional area controlled in accordance with signals received from a controller.
2. Description of the Related Art
It is well known that air or other gas flow and/or expansion in or from a closed system results in the production of sound. This may be a desirable outcome, and certain devices (e.g., musical instruments, sirens, etc.) are deliberately configured to produce an audible output. However, many other devices produce an audible output(s) as an unintentional side effect of their operation. Examples include air conditioning systems having fan or blower supplied airflow and intake systems for air compressors and internal combustion engines. Internal combustion engines are also well known to produce relatively loud and obtrusive exhaust noise due to the expansion of the heated gases used to produce the power output developed by the engine.
In many cases the audible output of the device is quite variable, depending upon the amount of air or gas flow through the device, among other factors. Generally speaking, the greater the air or gas flow through the device, the louder the sound output, although the operating frequency (e.g., the RPM of an internal combustion engine) and system resonance(s) also have a great deal of effect. In many cases, particularly in the field of musical instruments, the audible output may be varied in tone and/or intensity by varying the internal cross-sectional area of the instrument relative to the inlet and/or outlet cross-sectional areas.
In the above examples, where audible output is an undesired side effect of the operation of the device, e.g., in air compressors and engines, innumerable devices have been developed in the past to reduce the sound output of such devices. Most such devices are formed of relatively thin sheet metal and have a labyrinthine path therein for the air or gas to follow. Others rely upon some form of porous barrier that may also serve as a filtration system for incoming air to the system. Still others may utilize some form of active control, e.g., generating sound that is out of phase with the undesired sound output so that the generated sound substantially cancels the sound output of the device that produces the unwanted sound. Such systems not only require microphones to receive the sound output of the machine, but also require some form of audible output device (i.e., a speaker or speakers) to produce the out of phase audible signal to cancel the unwanted sound.
While such devices are effective to some degree, none have proven entirely satisfactory. Thus, an expandable chamber acoustic silencer solving the aforementioned problems is desired.